The present invention relates to measuring and positioning systems which can be used to improve the precision and accuracy with which the thickness of a plating layer can be measured.
In the past, X-ray thickness monitors have been used to measure the thickness of a plating layer such as a gold plating layer on components. U.S. Pat. No. 3,984,679 (Lublin) discloses one such thickness monitor.
The general approach used with such X-ray thickness monitors is to direct a small diameter X-ray beam onto the plating layer to be measured. The incident X-ray beam stimulates secondary X-ray emission which is then detected. The intensity of this secondary X-ray emission or X-ray fluorescence varies as a function of the thickness of the plating layer.
When this approach is used to measure the plating thickness layer of a moving terminal strip made up of an array of spaced terminals, two significant problems arise. The first is that individual terminals are separated by open space. Of course, this open space does not produce secondary X-rays characteristic of the plating layer, and the count of secondary X-rays detected during a measuring interval is therefore affected by the ratio between the width of the terminals and the width of the empty space between terminals.
In the past, this problem has been addressed by measuring the width of the empty space between adjacent terminals on a periodic basis and correcting the measured thickness as a function of the stored ratio between the width of the terminals and the width of the spacing between terminals. However, it has been found that this ratio varies significantly between terminals made with different dies and even between terminals made with a single die at different times. Furthermore, this approach cannot account for intermittent variations in the measurement duty cycle, caused by example by missing terminals. Thus, an occasional measurement of the ratio between the width of the terminals and the separation between adjacent terminals is not entirely satisfactory.
A second important problem in thickness measurements of the type described above is positioning the X-ray beam properly on the terminals. In modern plating operations for electrical terminals, plating layers such as gold layers are often applied only at desired regions of the termina. If the X-ray thickness measurement is not centered properly on these plating layers, the thickness measurement is of course inaccurate. Because the position of the desired measurement location is often not precisely controlled with respect to the carrier strip on which the terminals are mounted, optimum results cannot be achieved by simply positioning the X-ray beam at a predetermined separation from the carrier strip.